1. Field of Invention
This invention relates to linear, saturated polyester compositions containing sodium nitrite as a nucleating agent.
2. Description of the Prior Art
It is known to use nucleating agents in crystallizable polymers such as linear saturated polyesters of aromatic dicarboxylic acids. U.S. Pat. Nos. 3,435,093; 3,516,957 and 3,639,527; as well as Dutch Patent Numbers NL 7901605, and NL 7901609, disclose various approaches to molding thermoplastic compositions of linear saturated polyesters of aromatic dicarboxylic acids, and are particularly applicable to poly(ethylene terephthalate). These patents generally disclose the use of salts of hydrocarbon and polymeric carboxylic acids as nucleating agents for linear saturated polyesters.
A molding formulation based on a linear saturated polyester such as poly(ethylene terephthalate) should result in molded product having good physical properties including flexural strength, modulus, tensile strength and impact properties. Further, the molding compound should also have good molding properties including a fast molding cycle, a high melt flow index for sufficient flow into the mold, good mold release properties and good finished appearance.
From a practical point of view, it is desirable that satisfactory properties as outlined above be attained using water heated molds. These molds are heated to temperatures between about 76.7.degree. C. (170.degree. F.) to about 100.degree. C. (212.degree. F.). In order to accomplish this it is desirable for crystallization to begin at as high a temperature as possible upon the cooling of the polyester which was fed into the mold. T.sub.cc is used as a measurement to determine the temperature at which crystals first appear upon cooling from the melt. T.sub.cc can be measured using a Differential Scanning Calorimeter by the procedure detailed below.